Optical fiber and optical waveguide mode converters are well known and come in a variety of forms. They operate typically by transforming an input mode, usually a fundamental mode, into a higher order mode, or vice versa. An especially attractive mode converter device comprises a long period grating (LPG) formed in an optical fiber. See for example, U.S. Pat. No. 6,768,835, and T. Erdogan, “Fiber grating spectra,” J. Lightwave Technology vol. 15, p. 1277 (1997).
These mode converters operate with a single mode input, and typically a single mode output. Propagating light in more than one mode at a time, and controllably changing the mode of more than one mode at a time, would be an attractive goal, but to date not widely achieved.
The function of effective and controlled mode conversion is useful in devices that process optical signals in higher order modes (HOMs). See, for example, U.S. Pat. No. 6,768,835, issued to Siddharth Ramachandran on Jul. 24, 2004, and incorporated by reference herein. A problem in some devices of this kind is that the radial dependence of the E-fields of the HOMs in the HOM optical fiber is complicated and not very useful for applications that require specific E-field distributions. For instance, it may be desirable to deliver extremely high power laser energy through air-core photonic bandgap optical fibers (ACPBGs) or large mode area optical fibers (LMAs) as near Gaussian modes. However, such modes may have a very different E-field profile than the E-field profile within the HOM optical fiber. In another example, tight focusing of the output of the HOM optical fiber may be required. The desired beam shape for focusing is typically a Gaussian free-space mode. The beam shape exiting the HOM optical fiber is typically far from Gaussian.dA1/dz=iσA1+iκA1  Equation (5)